It is well recognized in the art of electrochemical cells to provide a cell having a cathode of divalent silver oxide (AgO). This material provides, when associated with a suitable anode, e.g., zinc and a suitable electrolye, e.g., aqueous potassium hydroxide containing or saturated with zinc oxide, a cell of relatively high coulometric capacity per unit volume and of relatively high voltage.
The art is also aware that AgO has certain deficiencies as a cathode material. Specifically, in storage and between periods of use, AgO in contact wih cell electrolyte tends to decompose releasing gaseous oxygen and thus simultaneously increases pressure inside a sealed cell and reduces coulometric capacity. Furthermore, AgO provides dual voltage when electrolytically discharged. Initially upon discharge, AgO provides a voltage characteristic of the AgO .fwdarw.Ag.sub.2 O reaction and thereafter provides a voltage characteristic of the Ag.sub.2 O .fwdarw.Ag reaction. Still further, a cell employing AgO as a cathode may be characterized by increasing internal resistance which increases with time at room and elevated temperatures. Heretofore, numerous disclosures have been made which teach solutions to one or more of these problems, including:
______________________________________ Inventor (or Country Patent No. Assignee) Issue Date ______________________________________ Japan 78/63543 Murakami et al. June 7, 1978 U.S. 3,017,448 Cahan January 1, 1962 U.S. 4,078,127 Megahed et al. March 7, 1978 U.S. 4,015,056 Megahed et al. March 29, 1979 ______________________________________
To the extent it is understood from the abstract printed in English in Chemical Abstracts, the Murakami et al. disclosure appears to be similar to that of Cohan in U.S. Pat. No. 3,017,448. Both Cahan and Murakami et al. disclose that the gassing of AgO can be inhibited by treatment of AgO with lead or a lead component. Cahan disclosed Pb.sup.o, PbO, Pb(OH).sub.2 or sodium plumbate as materials suitable for addition to an AgO electrode or treatment of an AgO electrode, while Murakami et al. teaches treating AgO with an alkaline solution containing a plumbate ion. Cahan specifically discloses that use of his treatment enhances or extends the duration of cell discharge at a voltage characteristic of the AgO .fwdarw.Ag.sub.2 O reaction. Thus, even though Cahan tended to solve the gassing problem associated with use of AgO cathodes, the solution exacerbated the aforestated dual voltage problem.
Mehahed et al., in U.S. Pat. No. 4,078,127 ("Megahed I"), claimed the use of metallic sulfide additives, included by dry mixing metallic sulfides and AgO, in AgO-containing cathodes. Specifically, Megahed I teaches that sulfides of cadmium, calcium, mercury, tin, tungsten, (or mixtures thereof) included in the cathode mixture, will inhibit AgO gassing. Megahed I disclosed that some of the sulfides, when included in the cathode mix, lowered cell impedance both initially and during discharge. Further, Megahed I disclosed that cadmium sulfide was the most effective sulfide additive under the conditions of mix preparation prescribed therein. However, under the conditions so prescribed, Megahed I disclosed that AgO cathode mixes which contained up to 3% lead sulfide were not found to be beneficial to the stability of such AgO electrochemical cells, since such cathode mixes did not prevent gassing and were detrimental to the impedance of cells upon high temperature storage. Moreover, Megahed I, while stating "cadmium sulfide was found to improve the voltage stability of cells...", does not disclose, much less claim, that cadmium sulfide additives yield cells with a single voltage discharges under low drain conditions.
Megahed et al., in U.S. Pat. No. 4,015,056 ("Megahed II"), discloses and claims a two-step process which yields an AgO cathodic material having a single voltage discharge. In the first step, an AgO pellet is treated with a mild reducing agent, such as methanol to form Ag.sub.2 O around the pellet. Then, after consolidation, a strong reducing agent, such as hydrazine, is applied to form a silver layer on the pellet surface. Cells containing such cathodes display the OCV and CCV values typical of the Ag.sub.2 O-Zn electrochemical cell system. However, such cells developed with time a relatively high impedance.
In addition to the aforelisted prior art, other prior art exists relative to the properties, formation and stability of AgO. "Electrode Phenomena of Silver-Silver Oxide System in Alkaline Batteries" by Shiro Yoshizawa and Zenichro Takehara published in the Journal of the Electrochemical Society of Japan, Volume 31, Number 3. pages 91-04 (1963) reports the effect of various metallic additives on the oxidation of silver electrodes. Among the additivies studied by the Japanese, was gold which was reported to increase the rate of formation of divalent silver oxide during the electrochemical formation of silver electrodes, i.e., oxidation of silver. Another article entitled "The Electric Resistivity of Silver Oxide" by Aladar Tvarusko published in the Journal of the Electrochemical Society, Volume 115, Number 11, pages 1105-1110 (November, 1968) reported on various metallic additives and their effect on the electric resistivity of divalent silver oxide. The article reports that mercury and lead added during the preparation of silver oxide decreased the electric resistivity of silver oxide. The article further confirms the disclosure of Cahan in U.S. Pat. No. 3,017,448 that the use of lead in an AgO cathode extends the time during dicharge at which the cathode exhibits voltaic properties characteristic of the AgO.fwdarw.Ag.sub.2 O reaction.
The patent literature also contains publications disclosing additives for alkaline batteries employing silver positive electrodes. U.S. Pat. No. 3,617,384 issued to Kamai et al. on Nov. 1, 1971 discloses a secondary zinc alkaline cell in which gold or silver, alloys thereof, oxides and hydroxides may be added to the zinc anode. U.S. Pat. No. 3,650,832 issued to Aladar Tvarusko on Mar. 21, 1972 discloses certain additives for divalent silver oxide selected from mercury, selenium, tellurium and combinations of mercury with tin or lead. Japanese Patent Application No. 48-1929 which was open to public inspection on Jan. 11, 1973 discloses adding gold hydroxide into the electrolyte of an alkaline silver oxide cell. U.S. Pat. No. 3,853,623 issued on Dec. 10, 1974 to Stuart M. Davis discloses gold ion additive for divalent silver oxide. U.S. Pat. No. 3,936,026 issued on Jan. 2, 1976 to Paul L. Howard and U.K. specification No. 1,474,895 disclose a cathode material which is a mixture of divalent silver oxide and sulfur and a cathode material which is a mixture of divalent silver oxide and silver sulfide. Great Britain Pat. No. 1,065,059 published Apr. 12, 1967 discloses the addition of cadmium oxide, magnesium oxide, or aluminum oxide to a silver electrode.
Even in view of the extensive developments in this field, there is a need for a more effective and more easily controllable means for providing a highly stable, single voltage-producing, low impedance-producing AgO cathode material. Therefore, it is an object of the present invention to provide for an AgO cathode material having a single voltage discharge at even low drains.
It is another object of this invention to provide for an AgO cathode material with low impedance.
Yet another object of this invention is to provide for an AgO cathode material which exhibits very little gassing after being placed in an electrochemical cell.
Another object of this invention is to provide a method of producing an AgO cathodic material which, after being placed in an electrochemical cell, provides for a cell which is thermally stable, has low impedance and displays a single voltage discharge at even low drains.
These and other subsidiary objectives which will appear are achieved by the practice of the present invention.